The optimum angles at which the inlet and exhaust valves of an internal combustion engine should open and close, both in relation to one another and in relation to the engine crankshaft, vary with the engine speed and load conditions. In an engine with a fixed valve timing, a compromise setting must be adopted in which different performance parameters are traded off one against the other.
To achieve improved performance over a range of engine speeds and loads, it has already been proposed to use a variable phase coupling to vary the phase of a camshaft in relation to the crankshaft and in relation to another camshaft.
Several variable phase couplings are known from the prior art, each having its own advantages and disadvantages. Noise and wear are particularly serious common problems that are caused by the fact that camshafts are subjected to torque reversal during operation. While a valve is being opened by a cam on the camshaft, torque has to be applied to the camshaft in one direction to overcome the resistance of the valve spring. On the other hand, while a valve is closing, its spring attempts to accelerate the camshaft and the camshaft experiences a torque reaction from the valve train acting in the opposite direction.
To suppress the noise resulting from torque reversals, it is necessary either to make the couplings very accurately or to employ some form of active backlash control. Such active backlash control conventionally contributes to an increase in sliding friction and increases the force required to bring about a change in phase. As a result, it is necessary to resort to a larger actuator and, if a hydraulic actuator is used, this also means a slower response because of the small diameter of the drillings in the camshaft that feed oil to the actuator.
A further problem with some known designs is that they cannot be retrofitted to an existing engine because they require major modification to the engine block, cylinder head or valve train.
With a view to mitigating the above problems, a variable phase coupling has already been proposed in the Applicants' co-pending International Patent Application PCT/GB98/02153, now WO 99/06675, published on Feb. 11, 1999, to provide which comprises a drive member for connection to the crankshaft having grooves of a first pitch, a driven member for connection to the engine camshaft having helical grooves of a different pitch facing towards the grooves in the drive member, balls engaged in the two helical grooves and serving to couple the drive and driven members for rotation with one another, an intermediate member disposed between the drive and driven members in contact with the balls, and means for displacing the intermediate member relative to the drive and driven members, the displacement of the intermediate member serving to move the balls relative to the helical grooves in the drive and driven members so as to vary the phase between the drive and driven members. In the latter co-pending patent application, the intermediate member has grooves on its inner and outer surfaces and two sets of balls are provided, the first set engaging in the pairs of helical grooves comprising the helical grooves in the driven member and the facing grooves on one surface of the intermediate member and the second set of balls engaging in the pairs of helical grooves that comprise the grooves in the drive member and the facing grooves on the other surface of the intermediate member.
The drive, driven and intermediate members in the latter proposal thus lie radially one inside the other thereby requiring the coupling to have a relatively large diameter. This can create packaging difficulties when there is insufficient space to accommodate a coupling of a large diameter.